Sintered metal carbides such as sintered silicon carbide are known to have superior characteristics as ceramic materials, etc.
Heretofore, metal carbides have been produced mainly by reacting metal elements or metal oxides and carbon under a strong heat. For example, a method in which one or more of, for example, such metal elements as silicon, titanium, tungsten, boron, aluminum, zirconium, hafnium, niobium, cobalt, molybdenum, tantalum, chromium and vanadium, or of oxides of these metals, and carbon are mixed and reacted under a strong heat, has already been practiced on an industrial scale. In this industrial method, the said mixture as it is, or together with an inert gas, e.g. argon or helium, is heated by using a high-frequency heating furnace, an Acheson type direct electric resistance furnace, or the like, to allow a reducing reaction and a carburizing reaction to take place, whereby there are obtained powdered metal carbides according to the starting materials used, e.g. SiC, TiC, WC, B.sub.4 C, ZrC, HfC, NbC, Mo.sub.2 C, TaC, Cr.sub.3 C.sub.2, VC, (SiTi)C, (WTi)C, (MoW)C, (WTiTaNb)C.
The thus obtained powders of metal carbides have a characteristic such that the finer the powders, the higher the strength of moldings obtained by subsequent sintering and the higher the sintering rate. This inevitably requires that the mixture of metal element or metal oxide and carbon, as an intermediate material, be of uniform and fine particles. In this connection, according to the prior art, the mixture in question is produced usually by pulverizing and mixing a coarse-grained or agglomerated metal element or metal oxide and carbon mechanically simultaneously and batchwise by means of a ball mill, a hammer mill, or the like. In this method, however, not only the material charging and product carrying out operations are troublesome because of the batch process, but also problems associated with working environment such as the formation of dust and the generation of noise during the pulverizing and mixing operation are unavoidable. Moreover, the mechanical pulverization requires a long time for obtaining a finely divided powder, and this long-time pulverization inevitably results in an increased amount of impurities incorporated which is ascribable to wear of the pulverizing machine itself, which in turn requires an impurity removing step as an after step such as chemical washing or adsorption. Because of these problems, it is next to impossible for the mechanical method to afford a finely divided mixture of below one micron.
There has also been known a method in which fine powders which have separately been obtained in advance by some suitable means are mixed together by using a mixer, a kneader, or the like. Even according to this method, however, biasing in the contents of the resultant mixture is unavoidable due to the difference in particle size, orientating property and specific gravity peculiar to each powder. In an effort to remedy this drawback, there has further been proposed a method in which a fine powder is dispersed in water in the form of colloid by means of a wet vibration mill or the like and then spray-dried by using a spray drier or the like. However, even according to this method, biasing in the mixed contents is apt to occur, and there is a fundamental problem such that because silicon oxide powder, titanium oxide powder and carbon powder which are aggregates with each individual particle being incapable of separating easily remain as aggregates as they are, it is difficult for them to constitute a mixture consisting of individual particles. Further, also from economic aspects such as equipment and energy costs, such conventional method is not desirable because it goes through fine powder manufacturing, wet mixing and spray drying steps.
On the other hand, as another method for obtaining metal carbides, U.S. Pat. No. 3,839,542 discloses a method of producing finely divided metal carbides directly by a vapor phase reaction from halides of metals such as boron, silicon, titanium, zirconium, hafnium, vanadium, tantalum, niobium, molybdenum, tungsten and thorium, and hydrocarbons in the presence of hydrogen. But this method cannot be considered economical in that it uses plasma for heating and requires both a large amount of hydrogen gas and a temperature of at least 1,300.degree. C.
Furthermore, in Japanese Patent Publication No. 36122/1981 there is disclosed a method of producing metal carbides directly by a vapor phase reaction from halides of metals selected from Group III through V metals of the Periodic Table, a carbon source and an anhydrous hydrogen halide. But this method cannot be considered economical, either, in that it requires both a large amount of hydrogen gas and the supply of an anhydrous hydrogen halide.
In the method of producing metal carbides directly by a vapor phase reaction according to the above-described prior arts, problems are also encountered in that the presence of oxygen and steam impedes the production of metal carbides, thus requiring control of the atmosphere and in that not a small quantity of metal halide vaporizes in gaseous state to the exterior of the reaction system.
It is the object of the present invention to overcome the above-mentioned problems of the prior art methods.
More particularly, the object of the present invention is to provide a process for once obtained a carbon-containing mixture as a precursor of metal carbide which mixture is extremely high in uniformity and consists of extremely fine particles, and thereafter obtaining a metal carbide of high quality by heat-treating the carbon-containing mixture.